Alumina (Al2O3) is one of the most popular ceramic materials in a diverse array of industrial applications and above all, nano-scale alumina powder exhibits many advantages, such as high surface area, low sintering temperature and excellent toughness, so it can be applied in sintered monolith, catalyst carriers, composite material fillers, paints, and even chemical mechanical polishing solutions. Thus, alumina has become an essential material in the modern industry.
There are many conventional methods for producing metal oxide powder, for example, directly smashing and grinding method, solid state reaction method, thermal decomposition method and so on. Such methods are characterized by smashing and grinding powder, and then sieving out the powder with specific particle size. For example, the solid state reaction method, which is applied maturely in the industry, is employed to mixing the material powder well, and the material powder is subjected to a thermal treatment to become a desired powder compound. After grinding and sieving the powder compound, the required powder with the desired particle size is obtained. However, those processes have the common bottleneck, for example, when the powder has the thinner particle size, it is more difficult to be grinded and brings more severe pollution during processing. Therefore, with respect to the powder having high purity or less than submicron scale, grinding steps are decreased in the recent process, such as the chemical method or the physicochemical method. In the chemical method, the required particle size is obtained by controlling the crystallite growth during the chemical precipitation process. In the physicochemical method, the required particle size is controlled by the physicochemical process.
The industrial alumina powder is mainly composed of a-phase alumina powder, which is obtained by thermally treating θ-phase alumina and transforming its phase. Bauxite the mixture of diaspore [AlO(OH)], gibbsite [Al(OH)3] and boehmite (AlOOH) called in the mineralogy serves as the starting material for producing the α-phase alumina powder. Aluminum hydroxide [Al(OH)3] crystal is then obtained after the mixture is dissolved and precipitated. Subsequently, the precipitated Al(OH)3 crystal is calcined to form alumina coarse-grained powder. The alumina coarse-grained powder is smashed and sieved to form various grades of industrial alumina powders.
In the thermal decomposition process, the α-phase alumina is mostly obtained by calcining the aluminum hydroxide, in which the calcinations temperature requires about 1000 degrees Celsius (° C.) to 1200° C. However, the aluminum hydroxide is subjected to dehydration and a series of phase transformations before obtaining the α-phase alumina. Below about 900° C., the χ-, η-, γ- and ρ-phases occur, and between about 900° C. and about 1150° C., δ-, κ- and θ-phases occur. Each transition phase mentioned as above depends on isomers of the aluminum hydroxide crystal. Finally, the α-phase alumina occurs. Among those, the θ-phase alumina powder is indeed the former transition phase of the α-phase alumina powder produced by the boehmite, so the process of the θ-phase alumina powder is the same as the α-phase alumina powder except for the process of the θ-phase alumina powder is carried out at lower temperature. However, more accompanying alumina transition phases present at the temperature as θ-phase alumina powder occurs, and they are difficult to be removed, resulting that it is complicated to obtain purer or single-phase θ-phase alumina powder. Therefore, the process cost is increased, and the θ-phase alumina powder is rarely applied in the industry.
All above the aforementioned description, there are many prior technologies to make efforts in solving the above problems. For example, U.S. Pat. No. 5,698,483 discloses a process for producing nano size powders comprising the steps of mixing an aqueous continuous phase comprising at least one metal cation salt with a hydrophilic organic polymeric disperse phase, forming a metal cation salt/polymer gel, and heat treating the gel at a temperature sufficient to drive off water and organics within the gel, leaving as a residue a nanometer particle-size powder.
U.S. Pat. No. 6,503,475 discloses a process for the production of ultrafine powders that includes subjecting a mixture of precursor metal compound and a non-reactant diluent phase to mechanical milling whereby the process of mechanical activation reduces the microstructure of the mixture to the form of nano-sized grains of the metal compound uniformly dispersed in the diluent phase. The process also includes heat treating the mixture of nano-sized grains of the metal compound uniformly dispersed in the diluent phase to convert the nano-sized grains of the metal compound into a metal oxide phase. The process further includes removing the diluent phase such that the nano-sized grains of the metal oxide phase are left behind in the form of an ultrafine powder.
U.S. Pat. No. 6,203,768 discloses a process for the production of ultrafine particles, which is based on mechanically activated chemical reaction of a metal compound with a suitable reagent. During mechanical activation a composite structure is formed which consists of an intimate mixture of nano-sized grains of the nano-phase substance and the reaction by-product phase. The step of removing the by-product phase, following mechanical activation, may involve subjecting the composite structure to a suitable solvent which dissolves the by-product phase, while not reacting with the solid nano-phase substance. The process according to the invention may be used to form ultrafine metal powders as well as ultrafine ceramic powders. Advantages of the process include a significant degree of control over the size and size distribution of the ultrafine particles, and over the nature of interfaces created between the solid nano-phase substance and the reaction by-product phase.
U.S. Pat. No. 6,521,016 discloses a method of producing nanophase Cu—Al2O3 composite powder by means of 1) the producing precursor powders by centrifugal spray drying process using the water base solution, in which Cu-nitrate (Cu(NO3)2 3H2O) and Al-nitrate (Al(NO3)3 9H2O) are solved to the point of final target composition (Cu-1 wt %/Al2O3), 2) the heat treatment process (desaltation process) at the 850 degrees C. for 30 min in air atmosphere to remove the volatile components such as the moisture and NO3 group in precursor powder and simultaneously to synthesize the nano CuO—Al2O3 composite powders by the oxidation of corresponded metal components and 3) the reduction heat treatment of CuO at 200 degrees C. for 30 min in reducing atmosphere to produce the final nanophase Cu—Al2O3 composite powders with the size below 20 nm.
U.S. Pat. No. 6,761,866 discloses a single step process for the synthesis of nanoparticles of phase pure ceramic oxides of a single or a multi-component system comprising one or more metal ions. The process comprises preparing a solution containing all the required metal ions in stoichiometric ratio by dissolving their respective soluble salts in an organic solvent or in water, preparing a precursor, adjusting the nitrate/ammonia content in the system, and heating the system.
U.S. Pat. No. 6,048,577 discloses nano-sized powders of alpha alumina can be obtained from a boehinite gel doped with a barrier-forming material such as silica that is then dried, fired and comminuted to powder form.
In a summary of the aforementioned methods, a substance different from the desired product is reacted by the chemical reaction or the mechanical strength to be a well dispersive precursor such as boehmite alumina, and the precursor is subjected to dehydration and phase transformation under a high temperature and/or a high pressure condition, so as to produce various aluminum oxide powders. However, such powders produced by the above methods have uneven particle sizes, and they still contain a certain amount of other transition phases, such as δ- or γ-phases. If it is desired to obtain purer or single-phase θ-phase alumina powder, the process is very complicated, and the cost is increased, too.
The θ-phase alumina is formed at the temperature lower than α-phase alumina, and is more stable to heat than α-phase alumina. Moreover, the θ-phase alumina has higher specific surface area ranging from 80 m2/g to 150 m2/g. Furthermore, the θ-phase alumina possesses less specific surface area decline under high temperature than the δ- or γ-phase alumina powders. Based on the above advantages, the θ-phase alumina will be more beneficially applied to the development of the high-temperature catalyst material than the δ- and γ-phase alumina powders in the future. Hence, there is a need for developing a method for producing nano-scale and highly pure θ-phase alumina microparticles, so as to overcome the problems of uneven particle sizes and less phase purity in the prior process.